[PATCH 4/5] drm/i915: Do not lie about atomic wait granularity

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Currently the wait_for_atomic_us only allows for a millisecond
granularity which is not nice towards callers requesting small
micro-second waits.

Re-implement it so micro-second granularity is really supported
and not just in the name of the macro.

This has another beneficial side effect that it improves
"gem_latency -n 100" results by approximately 2.5% (throughput
and latencies) and 3% (CPU usage). (Note this improvement is
relative to not yet merged execlist lock uncontention patch
which moves the CSB MMIO outside this lock.)

v2:
  * Warn when used from non-atomic context (if possible).
  * Warn on too long atomic waits.

v3:
  * Added comment explaining CONFIG_PREEMPT_COUNT.
  * Fixed pre-processor indentation.
  (Chris Wilson)

v4:
 * Commit msg update (gem_latency) and rebase.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/intel_drv.h | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index c2a62e9554b3..b6fcb5c800e7 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -44,6 +44,10 @@
  * contexts. Note that it's important that we check the condition again after
  * having timed out, since the timeout could be due to preemption or similar and
  * we've never had a chance to check the condition before the timeout.
+ *
+ * TODO: When modesetting has fully transitioned to atomic, the below
+ * drm_can_sleep() can be removed and in_atomic()/!in_atomic() asserts
+ * added.
  */
 #define _wait_for(COND, US, W) ({ \
 	unsigned long timeout__ = jiffies + usecs_to_jiffies(US) + 1;	\
@@ -66,8 +70,31 @@
 #define wait_for(COND, MS)	  	_wait_for((COND), (MS) * 1000, 1000)
 #define wait_for_us(COND, US)	  	_wait_for((COND), (US), 1)
 
-#define wait_for_atomic(COND, MS) 	_wait_for((COND), (MS) * 1000, 0)
-#define wait_for_atomic_us(COND, US)	_wait_for((COND), (US), 0)
+/* If CONFIG_PREEMPT_COUNT is disabled, in_atomic() always reports false. */
+#if defined(CONFIG_DRM_I915_DEBUG) && defined(CONFIG_PREEMPT_COUNT)
+# define _WAIT_FOR_ATOMIC_CHECK WARN_ON_ONCE(!in_atomic())
+#else
+# define _WAIT_FOR_ATOMIC_CHECK do { } while(0)
+#endif
+
+#define _wait_for_atomic(COND, US) ({ \
+	unsigned long end__; \
+	int ret__ = 0; \
+	_WAIT_FOR_ATOMIC_CHECK; \
+	BUILD_BUG_ON((US) > 50000); \
+	end__ = (local_clock() >> 10) + (US) + 1; \
+	while (!(COND)) { \
+		if (time_after((unsigned long)(local_clock() >> 10), end__)) { \
+			ret__ = -ETIMEDOUT; \
+			break; \
+		} \
+		cpu_relax(); \
+	} \
+	ret__; \
+})
+
+#define wait_for_atomic(COND, MS)	_wait_for_atomic((COND), (MS) * 1000)
+#define wait_for_atomic_us(COND, US)	_wait_for_atomic((COND), (US))
 
 #define KHz(x) (1000 * (x))
 #define MHz(x) KHz(1000 * (x))
-- 
1.9.1

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